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Euchambersia
Euchambersia was a therocephalian therapsid that lived during the Late Permian epoch, approximately 250 million years ago, in what is now South Africa. The genus contains a single species, E. mirabilis, named by paleontologist Robert Broom in 1931 from a skull missing the lower jaws; a second skull, belonging to an immature individual, was later described. It is a member of the family Akidnognathidae, which historically has also been referred by as the synonymous Euchambersiidae (named after Euchambersia). Euchambersia is notable among therocephalians for possessing ridged canines and a large indentation in the side of the skull. Under the erroneous assumption that the canines are grooved instead of ridged, it has been proposed that these structures supported a venom delivery mechanism. More recently, the internal structure of the skull of Euchambersia has been used as stronger evidence in favor of the hypothesis that it was venomous; other possibilities, such as the indentation supporting some sort of sensory organ, still remain plausible. Description Euchambersia was small and short-snouted for a therocephalian, with the type skull having a length of approximately 116 millimetres (4.6 in) once crushing and deformation of the fossil was reconstructed. The second skull belongs to a smaller individual, with a length of 80 millimetres (3.1 in); it was probably immature, judging by the lack of fusion in the skull. Teeth Although the skulls are incompletely preserved, CT scanning suggests that each premaxilla held five incisors, with the sockets becoming progressively larger from the first to the fifth incisor. Like other theriodonts, the crowns of the incisors are conical; they also lack serrations, unlike gorgonopsians and scylacosaurian therocephalians. The interior edge of the incisors seems to be slightly concave, and the back edge appears to have a ridge. The smaller specimen has an incisor preserved within its nasal cavity; it is more strongly recurved and has wear marks on its top edge, suggesting that it is probably a lower incisor. Its fourth incisor also has a replacement tooth growing behind it, accompanied by resorption of the root.1 The type specimen prominently preserves the right canine.1 It is large and round in cross-section,2 and bears a prominent ridge on the side of its front surface. Immediately beside this ridge is a shallow depression that becomes wider near the top of the tooth, which may have been interpreted as a groove by some authors.13 Theriodonts usually replace their teeth in an alternating4 (or distichial) pattern,56 such that the canine tooth is always functional; both skulls of Euchambersia show no sign of any replacement teeth developing, suggesting that Euchambersia' was reliant on having both canines present and functional simultaneously. Maxillary fossa and associated canals Behind the incisors and canines, there were no additional teeth in the jaw. Where teeth would be located in therocephalians that do have teeth behind the canines, there is instead a large depression, or fossa, in side of the maxilla. This fossa is 48% the length of the jaw in the type specimen, and 38% in the second skull. In both skulls, this fossa is divided into two parts: a shallower ridge on top, and a larger and deeper depression on the bottom. A wide furrow beginning behind the canine contacts the bottom of the fossa and then passes into the interior of the mouth. The bottom portion of the fossa is strongly pitted and bears a small opening, or foramen, on both the front and back surfaces.1 CT scanning shows that these openings lead to canals that connect to the trigeminal nerve, which controls facial sensitivity. The forward-directed canal also splits into the three main branches of the infraorbital nerve,7 all of which connect to the socket of the canine; the junction occurs about 3–6 millimetres (0.12–0.24 in) along the canal, another point of variation between the two skulls. The top branch, the external nasal ramus, splits into four branches in the type skull, but it does not split in the second skull. In other therapsids like ''Thrinaxodon, Bauria, and Olivierosuchus, the external nasal ramus generally splits into three or more branches. All of these canals would have brought nerves and nutrient-rich tissue to the root of the canines and the rest of the upper jaw. Discovery and naming The type specimen of Euchambersia was found on the South African farm of Vanwyksfontein, near the town of Colesberg.8 It consists of a single, distorted skull, catalogued as NHMUK R5696 and described by Robert Broom in 1931. A second, smaller skull, with the specimen number BP/1/4009, was later found in 19661 and described by Kitching in 1977.2 Both came from the same general layer of rock, in the Cistecephalus assemblage zone of the Beaufort Group within the Karoo Supergroup.2 This layer of rock has been dated to the Wuchapingian stage of the Late Permian,9 between 256.2 and 255.2 Mya. Classification Category:Permian synapsids Category:Therocephalia Category:Prehistoric synapsids of Africa Category:Therapsids